Indeed 2GB/month isn't a lot once you start using the internet for more than just browsing and e-mail. A few large downloads and some streaming radio and video could quite easily eat through that. Unfortunately there isn't a lot of scope for reducing the cost/GB under the current wholesale charges from BT, so if you're likely to download much more than 2GB/month regularly upgrading to the Active package would seem the obvious step.
Personally I find 20 GB is usually enough for me with plenty of general usage, a good bit of WoW playing a week and a reasonable chunk of large downloads.
Some months I use more than that, and some months I use a fair bit less. (fortunately as a Zen staff member I have a Pro account chucked in though so its less of a worry for me.)
I would guess that if you're not a heavy p2p or Newsgroup user 20GB will probably be enough, but of course I don't know how you use your own connection. If you log into the portal you can see more detailed usage (such as average per day, and a chart showing daily usage over the current month) Also there are some beta clients available for you to install on your computer which will show you at a glance how much is being used (see the sticky thread at the top of this forum for details)
Kindest regards,
James Sweet
http://www.zen.co.uk